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United States Patent |
5,209,206
|
Danno
,   et al.
|
May 11, 1993
|
Air-fuel ratio control system
Abstract
An air-fuel ratio control system compares an air-fuel ratio indicated by
air-fuel ratio information from an air-fuel ratio sensor and a target
air-fuel ratio determined depending on operating conditions of a motor
vehicle which incorporates the air-fuel ratio control system, for reliably
determining at least a failure of the air-fuel ratio sensor. When a
failure of the air-fuel ratio sensor is detected, an air-fuel ratio
feedback control process is stopped or the air-fuel ratio sensor is
disabled, preventing the air-fuel ratio from being corrected in error
based on an output signal from the air-fuel ratio sensor which has failed.
Therefore, the air-fuel ratio control system prevents problems such as
poor exhaust gas purification, reduced drivability, and unstable engine
idling from taking place.
Inventors:
|
Danno; Yoshiaki (Kyoto, JP);
Ishida; Tetsurou (Kyoto, JP);
Kodama; Yoshiaki (Kyoto, JP)
|
Assignee:
|
Mitsubishi Jidosha Kogyo Kabushiki Kaisha (Tokyo, JP)
|
Appl. No.:
|
727855 |
Filed:
|
July 10, 1991 |
Foreign Application Priority Data
| Jul 10, 1990[JP] | 2-182137 |
| Jul 17, 1990[JP] | 2-189227 |
Current U.S. Class: |
123/479; 123/672; 123/688 |
Intern'l Class: |
F02M 051/00 |
Field of Search: |
123/479,672
364/431.07
|
References Cited
U.S. Patent Documents
4938194 | Jul., 1990 | Kato et al. | 123/479.
|
4951632 | Aug., 1990 | Yakuwa et al. | 123/479.
|
4980834 | Dec., 1990 | Ikada et al. | 364/431.
|
5020499 | Jun., 1991 | Kojima et al. | 123/479.
|
Primary Examiner: Nelli; Raymond A.
Claims
What is claimed is:
1. An air-fuel ratio control system for an internal combustion engine on a
motor vehicle, comprising:
an air-fuel ratio sensor for producing an air-fuel ratio signal indicative
of the concentration of oxygen in an exhaust gas produced by a burned
air-fuel mixture in the internal combustion engine;
first means comprising failure determining means responsive to an output
signal from said air-fuel ratio sensor and a target air-fuel ratio
determined depending on operating conditions of the motor vehicle, for
determining at least an irreparable failure of said air-fuel ratio sensor
through comparison between said output signal and said target air-fuel
ratio;
air-fuel ratio feedback control means for correcting an air-fuel ratio
correction according to at least said air-fuel ratio signal so that an
actual air-fuel ratio of the internal combustion engine is equal to said
target air-fuel ratio; and
second means comprising failure processing means for disabling said
air-fuel ratio feedback control means and said air-fuel ratio sensor in
response to an output signal from said failure determining means which
indicates an irreparable failure of said air-fuel ratio sensor.
2. An air-fuel ratio control system according to claim 1, wherein said
first means further includes malfunction determining means responsive to
the output signal from said air-fuel ratio sensor and the target air-fuel
ratio, for determining a reparable malfunction of said air-fuel ratio
sensor, and said second means further includes malfunction processing
means for disabling said air-fuel ratio feedback control means in response
to an output signal from said malfunction determining means which
indicates a reparable malfunction of said air-fuel ratio sensor.
3. An air-fuel ratio control system according to claim 2, wherein said
first means further includes error calculating means for calculating an
error between said air-fuel ratio signal and said target air-fuel ratio,
said failure determining means comprising means for comparing said error
with a first preset value and determining a failure of said air-fuel ratio
sensor when said error exceeds said first preset value, and said
malfunction determining means comprising means for comparing said error
with a second preset value smaller than said first preset value and
determining a malfunction of said air-fuel ratio sensor when said error
exceeds said second preset value.
4. An air-fuel ratio control system according to claim 1, wherein said
air-fuel ratio sensor comprises a sensor cell for producing an output
electric signal depending on the difference between the concentration of
oxygen in the exhaust gas and the concentration of oxygen in a reference
gas having an oxygen excess ratio which is sufficiently larger than 1,
control means for detecting the output electric signal from said sensor
cell and producing an electric control signal to cause said output
electric signal to have a predetermined value, a pump cell for moving
oxygen ions in response to the electric control signal from said control
means, first detecting means for producing an air-fuel ratio signal
depending on an electric current flowing between said control means and
said pump cell, and second detecting means for producing a stoichiometric
air-fuel ratio signal corresponding to a voltage developed across said
pump cell, said failure determining means comprising means for comparing
at least one of said air-fuel ratio signal and said stoichiometric
air-fuel ratio signal with said target air-fuel ratio.
5. A method for controlling air-fuel ratio for an internal combustion
engine on a motor vehicle, comprising the steps of:
(a) producing an air-fuel ratio signal by an air-fuel ratio sensor
indicative of the concentration of oxygen in an exhaust gas produced by a
burned air-fuel mixture in the internal combustion engine;
(b) comparing an output signal from said air-fuel ratio sensor and a target
air-fuel ratio determined by operating conditions of the motor vehicle;
(c) determining at least an irreparable failure of said air-fuel ratio
sensor through the comparison at said step (b);
(d) correcting an air-fuel ratio correction according to at least said
air-fuel ratio signal so that an actual air-fuel ratio of the internal
combustion engine is equal to said target air-fuel ratio; and
(e) disabling said air-fuel ratio correction at said step (d) and said
air-fuel ratio sensor in response to an output signal from said step (c)
which indicates an irreparable failure of said air-fuel ratio sensor.
6. A method for controlling air-fuel ratio according to claim 5, wherein
said step (c) further includes the step of:
(c1) determining a reparable malfunction of said air-fuel ratio sensor and
said target air-fuel ratio, and
said step (e) further includes of the step of (e1) disabling said air-fuel
ratio correction at said step (d) in response to an output signal from
said step (c1) which indicates a reparable malfunction of said air-fuel
ratio sensor.
7. A method for controlling air-fuel ratio according to claim 6, wherein
said step (c) further includes the steps of:
(c2) calculating an error between said air-fuel ratio signal and said
target air-fuel ratio,
(c3) comparing said error with a first preset value, and
(c4) determining a failure of said air-fuel ratio sensor when said error
exceeds said first preset value, and
said step (c1) further includes the steps of
(c5) comparing said error with a second preset value smaller than said
first preset value, and
(c6) determining a malfunction of said air-fuel ratio sensor when said
error exceeds said second preset value.
8. A method for controlling air-fuel ratio according to claim 5, wherein
said step (a) further includes the step of:
(a1) producing an output electric signal by a sensor cell depending on a
difference between a concentration of oxygen in the exhaust gas and a
concentration of oxygen in a reference gas having an oxygen excess ratio
which is sufficiently larger than 1,
(a2) detecting said output electric signal from said sensor cell,
(a3) producing an electric control signal by control means to cause said
output electric signal to have a predetermined value,
(a4) moving oxygen ions by a pump cell in response to said electric control
signal produced at said step (a3),
(a5) producing an air-fuel ratio signal depending on an electric current
flowing between said control means and said pump cell, and
(a6) producing a stoichiometric air-fuel ratio signal corresponding to a
voltage developed across said pump cell, and
said step (c) further includes the step of comparing at least one said
air-fuel ratio signal and said stoichiometric air-fuel ratio signal with
said target air-fuel ratio.
9. A method for controlling air-fuel ratio for an internal combustion
engine on a motor vehicle, comprising the steps of:
(a) producing an air-fuel ratio signal by an air-fuel ratio sensor in
proportion to a concentration of oxygen in an exhaust gas produced by a
burned air-fuel mixture in the internal combustion engine;
(b) producing an output electric signal by a sensor cell depending on a
difference between a concentration of oxygen in a reference gas having an
oxygen excess ratio which is larger than 1;
(c) detecting said output signal produced at said step (b) from said sensor
cell;
(d) producing an electric control signal by control means to cause said
output electric signal to have a predetermined value;
(e) moving oxygen ions by a pump cell in response to said electric control
signal produced at said step (d);
(f) producing an air-fuel ratio signal depending on an electric current
flowing between said control means and said pump cell;
(g) producing a stoichiometric air-fuel ratio signal corresponding to a
voltage developed across said pump cell; and
(h) determining at least an irreparable failure of said air-fuel ratio
sensor through a comparison between an output signal from said air-fuel
ratio sensor and a target air-fuel ratio determined depending on operating
conditions of the motor vehicle, the comparison including the step of
comparing at least one said air-fuel ratio signal and said stoichiometric
air-fuel ratio signal with said target air-fuel ratio.
10. A method for controlling air-fuel ratio according to claim 9, further
including the step (i) determining a reparable malfunction of said
air-fuel ratio sensor responsive to the output signal from said air-fuel
ratio sensor and said target air-fuel ratio.
11. A method for controlling air-fuel ratio according to claim 10, further
including the step of:
(j) calculating an error between said air-fuel ratio signal and said target
air-fuel ratio,
(k) comparing said error with a first preset value,
(l) determining a failure of said air-fuel ratio sensor when said error
exceeds said first preset value,
(m) comparing said error with a second preset value smaller than said first
preset value, and
(n) determining a malfunction of said air-fuel ratio sensor when said error
exceeds said second preset value.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an air-fuel ratio control system for
controlling the air-fuel ratio (A/F) of an air-fuel mixture to be supplied
to an internal combustion engine.
2. Related Art
There has been proposed a linear A/F sensor utilizing the oxygen
concentration cell capability and oxygen ion pumping capability of
zirconia, for detecting whether the air-fuel ratio is on a leaner or
richer side of a stoichiometric ratio and also for detecting the value of
the air-fuel ratio (see Japanese Laid-Open Patent Publication No.
63(1988)-36140).
One conventional linear A/F sensor will be described below with reference
to FIGS. 17 through 20 of the accompanying drawings. FIG. 17 shows a
linear A/F sensor including a sensor cell 20 and a pump cell 21 which are
shown detached from each other, and each comprise a stabilized zirconia
device. The sensor cell 20 and the pump cell 21 are coupled to each other
through an insulation layer 22. The sensor cell 20 and the pump cell 21
have respective diffusion holes 23 and 24 defined therein for passing
therethrough exhaust gases from an internal combustion engine. The
insulation layer 22 has a detecting cavity 25 defined therein into which
exhaust gases, can be introduced through the diffusion holes 23 and 24 by
the sensor cell 20 and the pump cell 21. The diffusion holes 23 and 24 and
the detecting cavity 25 jointly serve as an element for controlling the
speed at which the exhaust gases are diffused. The insulation layer 22
also has a reference chamber 25a positioned below the detecting cavity 25
in spaced-apart relation thereto, where the reference chamber 25a is
defined between the sensor cell 20 and the pump cell 21. A reference gas
such as atmospheric air is introduced into the reference chamber 25a
through a communication hole (not shown). As shown in FIG. 18, the sensor
cell 20 has porous electrodes 26, 27 of platinum, and the pump cell 21 has
porous electrodes 28 and 29 of platinum, where the electrodes 26, 27, 28
and 29 double as catalysts. The sensor cell 20 has an electric heater 30
for heating itself to a temperature range, e.g.,
800.degree..+-.100.degree. C. in order to keep the sensor cell 20 active.
The sensor cell 20 functions as a conventional O.sub.2 sensor for
developing an electromotive force if there is an oxygen concentration
difference between the electrodes 26 and 27. The pump cell 21 also has the
same properties as the sensor cell 20, and serves to pump oxygen from a
negative electrode to a positive electrode when an electric current (pump
current Ip) is caused to flow between the electrodes 28 and 29.
A control assembly 31 detects an electromotive force Vs developed by the
sensor cell 20, and also controls the pump current Ip through a feedback
loop in order to keep constant the electromotive force Vs, i.e., in order
to keep an oxygen concentration corresponding to a stoichiometric ratio in
the cavity 25 or the diffusion holes 23 and 24. Since the pump current Ip
continuously varies with respect to the air-fuel ratio, as shown in FIG.
19, the air-fuel ratio can be calculated from the pump current Ip.
More specifically, the control assembly 31 includes a comparator 1 and an
integrator amplifier 2 with positive and negative power supplies. The
comparator 1 compares the electromotive force Vs and a reference voltage
Vref corresponding to the stoichiometric ratio. The output signal from the
comparator 1 is integrated by the integrator amplifier 2, whose integral
output signal is applied as the pump current Ip to the pump cell 21
through a resistor 5. At this time, a voltage drop across the resistor 5
is detected by a current detector 3 which produces a voltage signal
commensurate with the pump current Ip. Therefore, the pump current Ip is
detected indirectly by the current detector 3. The output signal of the
current detector 3 is applied to an adder 4 which then produces an output
signal Vout, in the range of 0 to 5 volts, as representing the air-fuel
ratio, according to the following equation:
Vout=G.multidot.Ip+Vstp,
where G is the current-to-voltage conversion gain of a current-to-voltage
converter which is composed of the resistor 5 and the current detector 3,
and Vstp is a step-up voltage in the range of 0 to 5 volts.
In the conventional system shown in FIG. 18, the voltage drop across the
resistor 5 is applied to a current inversion detector 6 to detect the
direction in which the pump current flows, thereby producing a
stoichiometric air-fuel ratio Vstc (see FIG. 20).
The air-fuel ratio of an internal combustion engine is controlled by a
feedback control loop so as to achieve a target air-fuel ratio based on
the air-fuel ration information produced by an air-fuel ratio sensor. For
example, when the air-fuel ratio is controlled within a narrow range or
within a window close to the stoichiometric air-fuel ratio, the three-way
catalytic converter in the exhaust system can operate highly efficiently.
With a lean-burn engine having a lean-NOx catalytic converter and a
three-way catalytic converter in the exhaust system, the air-fuel ratio is
controlled by a feedback control loop so as to achieve a target air-fuel
ratio, i.e., a certain leaner value, based on the air-fuel ratio
information from a linear A/F sensor.
Accurate control of the air-fuel ratio so that it reaches a target value
while the internal combustion engine is in operation is very important for
improved fuel economy, increased engine output power, a more stable idling
engine speed, purified exhaust emission, and improved drivability. It is
necessary that the linear A/F sensor which produces the air-fuel ratio
information be controlled so as not to be thermally deteriorated and
destructed due to blackening.
Air-fuel ratio sensors, particularly a linear A/F sensor, are complex in
structure, and should be composed of a heater, a sensor cell, and a pump
cell in combination for operation.
If the linear A/F sensor, or its pump cell, in particular, fails to
operate, then the air-fuel ratio signal Vout and the stoichiometric ratio
signal Vstc tend to deviate from their true values, and the air-fuel ratio
information produced by the linear A/F sensor becomes low in reliability.
Therefore, in the event of a failure of the linear A/F sensor, it is
desirable that the failure be detected early, the air-fuel ratio feedback
control process based on the sensor output be stopped, and another
air-fuel ratio control process be carried out instead.
It is also necessary for accurate air-fuel ratio control that the air-fuel
ratio information be stably produced at all times by the linear A/F
sensor.
The air-fuel ratio signal Vout produced by the linear A/F sensor poses no
problem insofar as the sensor operates in a stoichiometric air-fuel
mixture atmosphere. However, if the linear A/F sensor operates
continuously under a leaner air-fuel mixture atmosphere, then the air fuel
ratio signal Vout thereof is liable to vary with time as shown in FIG. 14.
More specifically, if the engine operates continuously with the air-fuel
ratio controlled for a certain leaner target air-fuel ratio, the air-fuel
ratio signal Vout produced by the linear A/F sensor tends to become lower
with time. It is known that when the engine is raced to shift the air-fuel
ratio temporarily toward a richer side, the pump current changes its
direction in the period ER (FIG. 14), and the air-fuel ratio then regains
the same value as that at the starting time ST, i.e., the O.sub.2
detecting characteristics are regarded as being recovered, at the end of
the period ER.
At the time the output signal from the linear A/F sensor indicates some
trouble, therefore, the air-fuel ratio information produced thereby
becomes less reliable.
In the event of a failure of the linear A/F sensor, therefore, it is
desirable to determine whether the sensor is being subjected to a
malfunction from which it can be recovered, or a failure from which it
cannot be recovered, so that any subsequent air-fuel ratio feedback
control process may be interrupted or another air-fuel ratio feedback
control process may be selected instead of the feedback control process.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide an air-fuel ratio
control system which is capable of reliably determining at least a failure
from which an air-fuel ratio sensor cannot be recovered, and of
subsequently carrying out a suitable process depending on the failure, for
thereby preventing various problems such as poor exhaust gas purification,
a reduction in drivability, and unstable engine idling.
According to the present invention, there is provided an air-fuel ratio
control system for an internal combustion engine on a motor vehicle,
comprising an air-fuel ratio sensor for producing an air-fuel ratio signal
indicative of the concentration of oxygen in an exhaust gas produced by a
burned air-fuel mixture in the internal combustion engine, and failure
determining means responsive to an output signal from said air-fuel ratio
sensor and a target air-fuel ratio determined depending on operating
conditions of the motor vehicle, for determining at least an irreparable
failure of said air-fuel ratio sensor through comparison between said
output signal and said target air-fuel ratio.
According to the present invention, there is also provided an air-fuel
ratio control system for an internal combustion engine on a motor vehicle,
comprising an air-fuel ratio sensor for producing an air-fuel ratio signal
indicative of the concentration of oxygen in an exhaust gas produced by a
burned air-fuel mixture in the internal combustion engine, first means
comprising failure determining means responsive to an output signal from
said air-fuel ratio sensor and a target air-fuel ratio determined
depending on operating conditions of the motor vehicle, for determining at
least an irreparable failure of said air-fuel ratio sensor through
comparison between said output signal and said target air-fuel ratio,
air-fuel ratio feedback control means for correcting an air-fuel ratio
correction according to at least said air-fuel ratio signal so that an
actual air-fuel ratio of the internal combustion engine is equalized to
said target air-fuel ratio, and second means comprising failure processing
means for disabling said air-fuel ratio feedback control means and said
air-fuel ratio sensor in response to an output signal from said failure
determining means which indicates an irreparable failure of said air-fuel
ratio sensor.
The air-fuel ratio control system compares an air-fuel ratio indicated by
air-fuel ratio information from an air-fuel ratio sensor and a target
air-fuel ratio determined depending on operating conditions of a motor
vehicle which incorporates the air-fuel ratio control system, for reliably
determining at least a failure of the air-fuel ratio sensor. When a
failure of the air-fuel ratio sensor is detected, an air-fuel ratio
feedback control process is stopped and the air-fuel ratio sensor is
disabled, and the air-fuel ratio is prevented from being corrected in
error based on an output signal from the air-fuel ratio sensor which has
failed.
The air-fuel ratio control system is therefore effective to prevent
problems such as poor exhaust gas purification, reduced drivability, and
unstable engine idling from taking place.
The above and other objects, features, and advantages of the present
invention will become apparent from the following description when taken
in conjunction with the accompanying drawings which illustrate preferred
embodiments of the present invention by way of example.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic block diagram of an air-fuel ratio control system
according to an embodiment of the present invention;
FIG. 2 is a detailed block diagram, partly in cross section, of the
air-fuel ratio control system shown in FIG. 1;
FIGS. 3(a) and 3(b) are diagrams showing a stoichiometric ratio signal Vstc
produced in the air-fuel ratio control system shown in FIG. 2;
FIGS. 4 through 7 are flowcharts of subroutines of a control program for
determining a failure of an air-fuel ratio sensor;
FIGS. 8(a) and 8(b) are a flowchart of a main routine of the control
program for determining a failure of an air-fuel ratio sensor, the control
program being executed by a controller in the air-fuel ratio control
system shown in FIG. 2;
FIG. 9 is a flowchart of a control program for calculating a rate of fuel
to be injected, the control programs being also executed by the controller
in the air-fuel ratio control system shown in FIG. 2;
FIG. 10 is a schematic block diagram of an air-fuel ratio control system
according to another embodiment of the present invention;
FIG. 11 is a block diagram, partly in cross section, of the air-fuel ratio
control system illustrated in FIG. 10;
FIG. 12 is a circuit diagram of a sensor driving circuit in the air-fuel
ratio control system shown in FIG. 11;
FIG. 13 is a diagram illustrative of various zones with respect to an
air-fuel ratio difference in the air-fuel ratio control system shown in
FIG. 11;
FIG. 14 is a diagram showing the output signal of a linear A/F sensor as it
varies with time;
FIGS. 15(a) and 15(b) are flowcharts of a main routine of a control program
for controlling an air-fuel ratio, the control program being executed by a
controller in the air-fuel ratio control system shown in FIG. 11;
FIGS. 15(c) and 16 are flowcharts of subroutines of the control program for
controlling an air-fuel ratio;
FIG. 17 is an exploded perspective view of a conventional air-fuel ratio
sensor;
FIG. 18 is a schematic view, partly in block form, of the conventional
air-fuel ratio sensor shown in FIG. 17;
FIG. 19 is a diagram showing the relationship between a pump current and an
air-fuel ratio; and
FIG. 20 is a diagram showing a stoichiometric ratio signal with its level
depending on the direction of the pump current.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
FIGS. 1 and 2 show an air-fuel ratio control system according to an
embodiment of the present invention.
As shown in FIG. 1, the air-fuel ratio control system generally includes an
air-fuel ratio sensor, a first unit including a failure determining
portion, and air-fuel ratio feedback control portion, and a second unit
including a failure processing portion.
The air-fuel ratio sensor includes a pump cell, a sensor cell, a
controller, a first air-fuel ratio detecting portion, and a second
air-fuel ratio detecting portion.
As shown in FIG. 2, the air-fuel ratio control system is disposed in a
control system for a fuel supply system for an internal combustion engine.
The control system for the fuel supply system calculates a rate of fuel to
be supplied to the engine based on air-fuel ratio (A/F) information
produced by a linear A/F sensor S which is positioned in an exhaust
passage of the engine, and the fuel supply system includes a fuel
injection nozzle N for injecting the calculated rate of fuel into an
intake passage of the engine.
The air-fuel ratio sensor includes the linear A/F sensor S, a control
assembly 31 connected as the controller means to the linear A/F sensor, a
current detector 3, an adder 4, and a current detecting resistor 5. The
air-fuel ratio sensor is of the same arrangement as that of the
conventional air-fuel ratio sensor shown in FIG. 18, and will not be
described in detail.
In FIG. 2, the control assembly 31 includes a comparator 1 and an
integrator amplifier 2 with positive and negative power supplies. The
comparator 1 compares an electromotive force Vs generated between the
electrodes 26 and 27 of the sensor cell 20 and a reference voltage Vref
such as of 0.4 V, for example. The output signal from the comparator 1 is
applied to and integrated by the integrator amplifier 2, whose positive or
negative control output signal is applied between the electrodes 28 and 29
of the pump cell 21 in order to supply a pump current Ip to the pump cell
21 so that the electromotive force Vs is equalized to the reference
voltage Vref (Vs=Vref).
The resistor 5 and the current detector 3 jointly serve as the first
air-fuel ratio detecting unit. Specifically, the current detector 3
detects the pump current Ip based on a voltage drop developed across the
resistor 5. The pump current IP, which bears air-fuel ratio information,
is converted by the adder 4 into an air-fuel ratio signal Vout in the
range of 0 to 5 volts. The air-fuel ratio signal Vout is then applied to
an engine controller 37.
A pump voltage Vp developed between the electrodes 28 and 29 of the pump
cell 21 is detected at a point A by the second air-fuel ratio detecting
unit, with the point A being on the line by which the pump current Ip is
supplied to the pump, cell 21. The second air-fuel ratio detecting unit
includes a buffer amplifier 8 connected to the point A, a CR filter 10, an
operational amplifier 11, a resistor 17, and two diodes 15 and 16.
More specifically, the pump voltage Vp is applied through a resistor 12 to
the inverting input terminal of the operational amplifier 11, whose output
signal is fed back to the inverting input terminal thereof through a
resistor 13. An upshifting voltage is applied through a resistive voltage
divider 14 to the noninverting input terminal of the operational amplifier
11. The two diodes 15 and 16, which are connected in series to each other,
are connected in a reverse-biased manner between a power supply of a
predetermined voltage and ground. The junction between the diode 15 and 16
is connected to the output terminal of the operational amplifier 11
through a resistor 17. With this arrangement, the second air-fuel ratio
detecting unit serves as an amplifier having a clipping capability. The CR
filter 10 serves to prevent a current surge and remove noise.
Basically, the pump voltage Vp has a characteristic curve, as shown in FIG.
3(a), which jumps or increases suddenly at the stoichiometric air-fuel
ratio. Since the electromotive force of the pump cell overlaps the, pump
voltage Vp, the second air-fuel ratio detecting unit produces, as its
output signal, a stoichiometric air-fuel ratio signal Vstc which has
different levels on leaner and richer sides of the stoichiometric air-fuel
ratio. The stoichiometric air-fuel ratio signal Vstc is applied to the
engine controller, 37. Since the second air-fuel ratio detecting unit is
mainly composed of the operational amplifier 11, the stoichiometric
air-fuel ratio signal Vstc has a relatively smooth waveform as shown in
FIG. 3(b). The second air-fuel ratio detecting unit therefore has output
characteristics which are similar to those of a stoichiometric air-fuel
ratio sensor.
The linear A/F sensor S has an electric heater 30 for heating itself, with
the electric heater 30 being connected to a heated driver 32. The heater
driver 32 includes a conventional bridge circuit or the like (not shown)
for keeping a heater resistance RH at a preset value.
The sensor cell 20 is also connected to a detecting circuit 38 for
detecting the electromotive force generated by the sensor cell 20 and
producing an output signal Vs corresponding to the detected electromotive
force.
A pump current cutting circuit 39 is connected to the input terminal of the
integrator amplifier 2 and ground, for example. Thus, in response to a
pump current cutting signal from the controller 37, the pump current
cutting circuit 39 cuts off the pump current Ip by grounding the input
terminal of the integrator amplifier 2.
A starter switch 40 is disposed in a combination switch assembly (not
shown) of the engine, and applies an ON or OFF signal to the controller
37.
The controller 37 is mainly composed of a microcomputer, and includes
drivers 371 and 372, an input/output interface 373 for receiving various
output signals and applying control signals to the drivers 371 and 372, a
memory 374 which stores a control program for determining a failure (see
FIGS. 4 through 8(a) and 8(b)), a control program for calculating a rate
of fuel to be injected (see FIG. 9) and various characteristic data and
values, and a control unit 375 for calculating control values according to
the control programs.
The functions of the controller 37 will be described below with reference
to FIG. 1. The controller 37 has the first unit which includes the failure
determining portion, as described above. The failure determining portion
receives a signal from at least one of the first air-fuel ratio detecting
portion which produces the air-fuel ratio signal Vout depending on the
control current from the controller, and the second air-fuel ratio
detecting portion which produces the stoichiometric air-fuel ratio signal
Vstc in response to the detected control voltage that is applied to the
pump cell by the controller. The failure determining portion then compares
the received signal with a target air-fuel ratio signal to determine
whether the compared signal falls within an allowable range. If the
compared signal does not fall within the allowable range, then the failure
determining portion determines that the air-fuel ratio sensor has failed.
The controller 37 also has the second unit which includes the failure
processing portion, and the air-fuel ratio feedback control portion which
effects feedback control on the rate of fuel to be injected based on the
air-fuel ratio signal. In response to a failure signal from the first
unit, the failure processing portion stops the air-fuel ratio control
process which is being carried out by the air-fuel ratio feedback control
portion, in cooperation with the pump current cutting circuit 39. As
described later on, when the pump current cutting circuit 39 is energized,
the air-fuel ratio sensor produces a quasi-signal indicating that the
detecting cavity is kept in a stoichiometric air-fuel mixture atmosphere.
A process of determining a failure of the air-fuel ratio sensor and a
process of calculating a rate of fuel to be injected into the engine,
which are carried out at the same time that the rate of fuel to be
injected is controlled under air-fuel ratio feedback control and open-loop
control processes by the controller 37, will be described below with
reference to the flowcharts of FIGS. 4 through 9.
The failure determining process is effected according to the control
program shown in FIGS. 4, 5, 6, 7, 8(a) and 8(b). The control program has
a main routine shown in FIGS. 8(a) and 8(b). In the main routine, the
heater 30 is turned on in a step al, which is followed by a step a2 that
determines whether a starter flag is 1 or not. If the starter flag is not
1, then control jumps to a step a8, and if the starter flag is 1, then
control proceeds to a step a3. The starter flag is set when the starter
switch SW is turned on, as shown in FIG. 7.
The starter flag is cleared to 0 in the step a3, and fuel determining flags
F1 and F2 and a pump cell operation flag that allows the pump current Ip
to be supplied are cleared in respective steps a4 and a5. In a step a6, a
sensor starting timer is reset which defines a time to start the linear
A/F sensor S. Thereafter, the sensor starting timer is started in a step
a7. Consequently, the sensor starting timer counts the period of time from
the time when the starter switch SW changes from the OFF state to the ON
state. A next step a8 determines whether the count of the sensor starting
timer exceeds a preset value .theta. which has been set to an interval of
time long enough for the air-fuel ratio sensor to be activated while the
engine is being warmed up. If the count of the sensor starting timer does
not exceed the preset value .theta., then control goes to a step a15 in
which the air-fuel ratio feedback control process is inhibited. Then, the
open-loop control process is effected in a step a16, i.e., a rate of fuel
to be injected is determined from a predetermined map based on the engine
rotational speed and the engine load, and the determined rate of fuel is
stored in a predetermined memory area. Thereafter, control goes back from
the step a16 to the step al.
Concurrent with this, a fuel injection routine (not shown) is executed in
response to an interrupt at a given crankshaft angle, for thereby
injecting fuel to achieve a predetermined target air-fuel ratio.
Thereafter, since the starter flag is 0 in the step a2, control goes from
the step a2 directly to the step a8. If the count of the sensor starting
timer exceeds the preset value .theta. in the step a8, then control
proceeds to a step a9. In the step a9, if the sensor starting timer is
still in operation, the counting operation thereof is stopped while
retaining the count achieved so far. Then, control goes from the step a9
to a step a10.
The step a10 determines whether the pump cell operation flag is 1 or not.
If the pump cell operation flag is not 1, then control proceeds to a step
all in which the pump cell 21 is operated. Then, the pump cell operation
flag is set to 1 in a step a12, which is followed by a step a13 in which a
pump cell operation timer is started. A step a14 determines whether the
count of the pump cell operation timer exceeds a preset value .epsilon.
which has been set to an interval of time long enough for the output
signal of the air-fuel ratio sensor to be stabilized. If the count of the
pump cell operation timer does not exceed the preset value .epsilon., then
control goes to the step a15 for continuing the open-loop control process.
If the count of the pump cell operation timer exceeds the preset value
.epsilon., i.e., if the preset wait time has elapsed and the pump current
Ip becomes reliable, then control goes from the step a14 to a step a17. In
the step a17, if the pump cell operation timer is still in operation, the
counting operation thereof is stopped while retaining the count achieved
so far. Then, control goes from the step a17 to a step a18.
The step a18 and subsequent steps a19 and a20 determine whether the linear
A/F sensor S has failed or not.
The step a18 is shown as a subroutine #1 in FIG. 4. The subroutine #1
determines whether the linear A/F sensor S has failed or not based on the
air-fuel ratio signal Vout. If the fuel determining flag F1 is not 1 in a
step b1 and the air-fuel ratio feedback control process is effected in a
step b2, then control goes to a step b3. If the fuel determining flag F1
is 1 in the step b1 and the air-fuel ratio feedback control process is not
effected in the step b2, then control returns to the main routine shown in
FIGS. 8(a) and 8(b).
The step b3 reads a target air-fuel ratio that has already been determined
in the main routine depending on operating conditions of the motor vehicle
which incorporates the air-fuel ratio control system according to the
present embodiment. Then, the air-fuel ratio signal Vout from the linear
A/F sensor S is read in a step b4. A step b5 thereafter converts the
air-fuel ratio signal Vout into an actual air-fuel ratio according to a
predetermined map (not shown) of air-fuel ratios vs. air-fuel ratio
signals.
A step b6 calculates a deviation or error .DELTA.A/F between the target
air-fuel ratio and the detected air-fuel ratio from the air-fuel ratio
sensor. A step b7 then determines whether the error .DELTA.A/F exceeds a
preset value .alpha. for determining a sensor failure or not. If the error
.DELTA.A/F does not exceed the preset value .alpha., then control returns
to the main routine. If the error .DELTA.A/F exceeds the preset value
.alpha., then the fuel determining flag F1 is set to 1 in a step b8.
Thereafter, control returns to the main routine.
The step a19 is shown as a subroutine #2 in FIG. 5. The subroutine #2
determines whether the linear A/F sensor S has failed or not based on the
stoichiometric air-fuel ratio signal Vstc. If the fuel determining flag F2
is not 1 in a step cl and the air-fuel ratio feedback control process is
effected in a step c2, then control goes to a step c3. If the fuel
determining flag F2 is 1 in the step c1 and the air-fuel ratio feedback
control process is not effected in the step c2, then control returns to
the main routine shown in FIGS. 8(a) and 8(b).
The step c3 reads the target air-fuel ratio that has already been
determined in the main routine depending on operating conditions of the
motor vehicle. Then, a step c4 determines whether the target air-fuel
ratio is close to the stoichiometric air-fuel ratio (i.e., falls in a
range indicated by e in FIG. 3). If the target air-fuel ratio is close to
the stoichiometric air-fuel ratio, then control returns to the main
routine, and if not, then control proceeds to a step c5.
The step c5 determines whether the target air-fuel ratio is richer than the
stoichiometric air-fuel ratio. If the target air-fuel ratio is richer than
the stoichiometric air-fuel ratio, then control goes to a step c6, and
otherwise, control goes to a step c8.
The step c6 reads the present stoichiometric air-fuel ratio signal Vstc,
and a subsequent step c7 determines whether the read stoichiometric
air-fuel ratio signal Vstc indicates a richer value. If the signal Vstc
indicates a richer value in the step c7, then control returns to the main
routine. If the stoichiometric air-fuel ratio signal Vstc indicates a
leaner value in the step c7, then/since the target air-fuel ratio is
richer than the stoichiometric air-fuel ratio, and hence the target and
stoichiometric air-fuel ratios do not agree with each other, it is
determined that the air-fuel ratio sensor is suffering some failure, and
control goes to a step c10 in which the fuel determining flag F2 is set to
1. Thereafter, control returns to the main routine. The step c8 reads the
present stoichiometric air-fuel ratio signal Vstc, and a subsequent step
c9 determines whether the read stoichiometric air-fuel ratio signal Vstc
indicates a richer value. If the signal Vstc indicates a leaner value in
the step c9, then control returns to the main routine. If the
stoichiometric air-fuel ratio signal Vstc indicates a richer value in the
step c9, then, since the target air-fuel ratio is leaner than the
stoichiometric air-fuel ratio, and hence the target and stoichiometric
air-fuel ratios do not agree with each other, it is determined that the
air-fuel ratio sensor is suffering some failure, and control goes to the
step c10 in which the fuel determining flag F2 is set to 1. Thereafter,
control returns to the main routine.
The step a20 is shown as a subroutine #3 in FIG. 6. The subroutine #3
determines whether the linear A/F sensor S has failed or not based on the
electromotive force Vs generated by the sensor cell 20. If the fuel
determining flag F1 is not 1 in a step d1, then control goes to a step d2.
If the fuel determining flag F1 is 1 in the step d1, then control returns
to the main routine shown in FIGS. 8(a) and 8(b)
In the step d2, the electromotive force Vs generated by the sensor cell 20
is detected by the detecting circuit 38. Then, a step d3 determines
whether the detected electromotive force Vs, which may be set to 450 mV,
for example, falls within an allowable range of from .phi. to .psi. or
not. The allowable range has experimentally been determined in advance. If
the electromotive force Vs falls within the allowable range in the step
d3, then control returns to the main routine. If the electromotive force
Vs falls outside of the allowable range, then it is determined that the
sensor cell 20 is being subjected to some failure, and the fuel
determining flag F1 is set to 1 in a step d4. Thereafter, control goes
back to the main routine.
After the subroutines #1, #2, #3 in the steps a18, a19 and a20, control
goes to a step a21 in the main routine. The step a21 determines whether
the fuel determining flag F1 is 0 or not. If the fuel determining flag F1
is not 0, then a pump cell operation stop signal is applied through the
driver 372 to the pump current cutting circuit 39 to cut off the pump
current Ip in a step a22, for thereby preventing the pump cell 21 from
being blackened. Then, control proceeds from the step a22 to the step a15
for the air-fuel ratio open-loop control process.
If the fuel determining flag F1 is 0, then control proceeds from the step
a21 to a step a23. The step a23 determines whether the present operating
conditions of the motor vehicle fall within an air-fuel ratio feedback
control range or not. If the present operating conditions are not in the
air-fuel ratio feedback control range, then control goes to the step a15
for the air-fuel ratio open-loop control process.
If the present operating conditions of the motor vehicle are in the
air-fuel ratio feedback control zone in the step a23, then control goes to
a step a24. The step a24 determines whether the target air-fuel ratio in
the present operating conditions is the stoichiometric air-fuel ratio or
not. If the target air-fuel ratio is the stoichiometric air-fuel ratio,
then control goes to a step a26. If the target air-fuel ratio is not the
stoichiometric air-fuel ratio, i.e., is on the leaner or richer side of
the stoichiometric air-fuel ratio, then control goes to a step a25.
The step a26 determines if the fuel determining flag F2 is 1 or not. If the
fuel determining flag F2 is not, then control goes to a step a27 in which
the a ratio feedback control process is carried out to achieve the
operation of the engine at the stoichiometric air-fuel ratio, based on the
stoichiometric air-fuel ratio signal Vstc according to a routine for
calculating a rate of fuel to be injected as shown in FIG. 9. Thereafter,
control goes back to the step a1.
If the fuel determining flag F2 is 1 in the step a26, indicating that the
stoichiometric air-fuel ratio signal vstc is abnormal, control goes to the
step a15 for the air-fuel ratio open-loop control process.
In the step a25, the air-fuel ratio feedback control process is carried out
to achieve the target air-fuel ratio (on the leaner or richer side of the
stoichiometric air-fuel ratio), based on the air-fuel ratio signal Vout
according to the routine shown in FIG. 9. Thereafter, control returns from
the step a25 to the step a1.
The routine shown in FIG. 9 will be described below. First, a step el
determines whether a condition to start a fuel injection feedback control
process is met or not, based on an input signal from a conventional
detection.
If the condition is not met, then control goes to a step e2, and if the
condition is met, then control goes to a step e3 for the air-fuel ratio
feedback control process.
In the step e2, a fuel injection rate corrective coefficient KFs is set to
1. As a result, the engine is continuously operated to equalize the
air-fuel ratio to the stoichiometric air-fuel ratio according to the
open-loop control process. Then, control proceeds to a step e4 in which a
fuel injection rate Fuel is calculated. Specifically, an interrupt routine
is effected to read an engine rotational speed N from an engine rotational
speed sensor 41, a rate A/N of intake air from the engine rotational
sensor 41 and an air flow sensor 42, and atmospheric pressure data from an
atmospheric pressure sensor 43. A basic fuel injection rate F(A/N,N) is
calculated from the air intake rate A/N and the engine rotational speed N.
The calculated basic fuel injection rate F(A/N,N) is multiplied by the
corrective coefficient KFB (described later on) and another corrective
coefficient K depending on a parameter such as the atmospheric pressure,
thus obtaining the fuel injection rate Fuel. Thereafter, control returns
from the step e4 to the main routine.
Data, such as the air intake pressure, the throttle opening, or the like,
may be employed instead of the intake air rate A/N.
If the condition to start the fuel injection is met in the step e1, then
the step e3 determines whether an average value .DELTA.V.sub.M of errors
or differences .DELTA.V between preset and actual stoichiometric air-fuel
ratios is to be cleared or initialized. If the average value .DELTA.VM is
to be cleared, then the averaged value .DELTA.V.sub.m is cleared in a step
e5, which is then followed by a step e6.
The step e6 reads the stoichiometric air-fuel ratio signal Vstc and the
air-fuel ratio signal Vout.
A step e7 compares the read value of Vstc with the value in the previous
cycle, and determines whether they differ from each other, i.e., whether
the stoichiometric air-fuel ratio signal Vstc has changed between a high
level VHi and a low level VLo (see FIG. 20). If the stoichiometric
air-fuel ratio signal Vstc has changed in level because the present
air-fuel ratio has reached the stoichiometric air-fuel ratio, then control
goes to a step e8, and if the stoichiometric air-fuel ratio signal
V.sub.stc has not changed, then control jumps to a step e9.
The step e8 determines whether conditions for correcting the error average
.DELTA.V.sub.M are satisfied (e.g., if the accelerator or throttle opening
has changed by a value less than or equal to a reference value or, if the
target air-fuel ratio has been modified immediately before, etc.). If the
condition for correcting the error average .DELTA.V.sub.M are met, then
control goes to a step e10, and if the conditions for correcting the error
average .DELTA.V.sub.M are not met, control goes to the step e9.
In the step e10, the air-fuel ratio signal Vout at the time it has reached
the stoichiometric air-fuel ratio is stored as an actual value Vst. Then,
an error or difference .DELTA.V is calculated between the actual air-fuel
ratio Vst and a predetermined stoichiometric air-fuel ratio Ust, and an
average value .DELTA.V.sub.M of the present and previous errors or
differences is calculated in order to eliminate disturbances, so that the
average value .DELTA.V.sub.M is updated.
The step e9 calculates the corrective coefficient K.sub.FB for the fuel
rate. Specifically, the air-fuel ratio signal Vout at the time is
corrected by the error average .DELTA.V.sub.M, thereby producing an
air-fuel ratio indicated by (A/F).sub.2 =f(Vout-.DELTA.V.sub.M), for
example.
Then, the target air-fuel ratio A/F that has already been determined in the
main routine depending on operating conditions of the motor vehicle is
read, and an error or difference .epsilon. between the read target
air-fuel ratio A/F and the actual air-fuel ratio (A/F).sub.2 is
calculated, and a difference .DELTA..epsilon. between the presently
calculated error .epsilon. and the previously calculated error is also
calculated. Finally in the step e9, a corrective coefficient K.sub.FB is
calculated for the control of a fuel injection rate based on the air-fuel
ratio.
The corrective coefficient KFs is calculated as the sum of, or difference
between, a proportional term K.sub.A (.epsilon.) of, a gain depending on
the level of the error .epsilon., an offset K.sub.p for the prevention of
a response delay from the three-way catalytic converter, a differential
term K.sub.D(.DELTA..epsilon.) depending on the difference
.DELTA..epsilon., an integral term .SIGMA.K.sub.I(.epsilon.,tFB), and 1.
Thereafter, control goes to the step e4 in which a proper rate of fuel to
be supplied at the time is calculated from the corrective coefficients
K.sub.FB, K, and the basic fuel injection rate F(A/N,N). Control then
returns to the main routine.
The rate of fuel to be supplied which is thus determined in the routine
shown in FIG. 9 is called in the fuel injection routine that is executed
at the time of an interrupt effected in response to a crankshaft angle
signal produced in the main routine. The fuel injection nozzle N is then
actuated by the driver 371 for an interval of time corresponding to the
determined rate of fuel to be supplied, thereby injecting fuel at the rate
which achieved the desired air-fuel ratio.
In the above embodiment, the first and second detecting portion apply the
air-fuel ratio signal and the stoichiometric air-fuel ratio signal,
respectively, to the controller which has the comparator. However, only
one of the air-fuel ratio signal and the stoichiometric air-fuel ratio
signal may be applied to the comparator for determining a failure. This
alternative results in a simpler system arrangement.
FIGS. 10 and 11 show an air-fuel ratio control system according to another
embodiment of the present invention.
As shown in FIG. 11, the air-fuel ratio control system is disposed in a
fuel supply system for an internal combustion engine 10. The fuel supply
system calculates a rate of fuel to be supplied to the engine based on
air-fuel ratio (A/F) information produced by a linear A/F sensor S which
is positioned in an exhaust passage 11 of the engine 10, and includes a
fuel injection nozzle N for injecting the calculated rate of fuel into an
intake passage 13 of the engine 10.
The linear A/F sensor S and the control assembly 31 therefor shown in FIG.
11 are of the same arrangement as those of the conventional system shown
in FIG. 18, and will not be described in detail.
In FIG. 11, the linear A/F sensor S applies an air-fuel ratio signal Vout,
in the range of 0 to 5 volts, to an engine controller 12. The control
assembly 31 for the linear A/F sensor S has a pump current cutting circuit
14 connected as shown in FIG. 12, with the pump current cutting circuit 14
serving as part of a failure processing portion.
As shown in FIG. 12, the pump current cutting circuit 14 includes a
transistor 15 whose base can be supplied with a pump current cutting
signal from the controller 12. When the pump current cutting signal is
applied to the base of the transistor 15, the junction between a
comparator 1 and an integrator amplifier 2 with positive and negative
power supplies is brought to a potential of 0. Therefore, a pump current
Ip becomes zero, so that the comparator 1 produces an output signal as if
the stoichiometric air-fuel ratio were detected.
A starter switch 16 is disposed in a combination switch assembly (not
shown) of the engine, and applies an ON or OFF signal to the controller 12
as shown in FIG. 11. An air flow sensor 17 applies a signal indicative of
intake air rate information to the controller 12. An engine rotational
speed sensor 18 applies a signal indicative of engine rotational speed
information to the controller 12. An atmospheric pressure sensor 19
applies a signal indicative of atmospheric pressure information to the
controller 12.
The controller 12 includes microcomputer, drivers 121 and 122, an
input/output interface 123 for receiving various output signals and
applying control signals to the drivers 121 and 122, a memory 124 which
stores a control program for controlling the air-fuel ratio (see FIGS.
15(a) through 15(c)) and various threshold values, and a control unit 125
for calculating control values according to the control program.
The functions of the controller 12 will be described below with reference
to FIG. 10. The controller 12 has a first unit including a deviation
calculating portion responsive to an air-fuel ratio signal Vout from the
control assembly 31 for the linear A/F sensor S, for calculating an error
or difference .DELTA.A/F between an actual air-fuel ratio according to the
air-fuel ratio signal Vout and a target air-fuel ratio which is preset
depending on operating conditions of the motor vehicle which incorporates
the air-fuel ratio control system. The first unit also has a failure
determining portion and a malfunction determining portion. The controller
12 also has a second unit including a malfunction processing portion and a
failure processing portion, and an air-fuel ratio feedback control portion
for effecting feedback control on the air-fuel ratio based on the air-fuel
ratio signal.
The malfunction determining portion produces a malfunction signal if the
error .DELTA.A/F exceeds a threshold value .pi.. The failure determining
portion produces a failure signal if the error .DELTA.A/F exceeds another
threshold value .alpha.. The malfunction processing portion interrupts the
air-fuel ratio feedback control process based on the air-fuel ratio
signal, when the malfunction signal is produced by the malfunction
determining portion. The failure processing portion interrupts the
air-fuel ratio feedback control process based on the air-fuel ratio signal
and stops sensor operation, when the failure signal is produced by the
failure determining portion.
As shown in FIG. 13, the threshold value .alpha. is greater than the
threshold value .pi..
The threshold value .pi. is determined in view of a reduction in level of
the air-fuel ratio signal Vout which takes place with time when the engine
continuously operates with a lean air-fuel mixture (during an interval EN
in FIG. 14). When the threshold value .pi. is exceeded, the air-fuel ratio
feedback control process is interrupted, but the linear A/F sensor is
allowed to operate. The other threshold value .alpha., which is greater
than the threshold value .pi., is selected to be of such a level that if
the error .DELTA.A/F exceeds the threshold value .alpha., it is determined
that the air-fuel sensor has failed and cannot be recovered from the
failure.
A process of controlling an air-fuel, ratio with the air-fuel ratio control
system shown in FIGS. 10 through 12 will be described with reference to
the control program shown in FIGS. 15(a) through 15(c) and 16. The
air-fuel ratio control process is carried out simultaneously with a
process of controlling the rate of fuel to be injected (through air-fuel
ratio feedback control and air-fuel ratio open-loop control) with the
controller 37.
The control program has a main routine shown in FIGS. 15(a) and 15(b). In
the main routine, the heater 30 (see FIG. 18) is turned on in a step fl,
which is followed by a step f2 that determines whether a starter flag is 1
or not. If the starter flag is not 1, then control jumps to a step f8, and
if the starter flag is 1, then control proceeds to a step f3. The starter
flag is set when the starter switch SW is turned on, as shown in FIG.
15(c).
The starter flag is cleared to 0 in the step f3, and a fuel determining
flag F1, and a malfunction flag F2, and a pump cell operation flag that
allows the pump current Ip to be supplied are cleared in respective steps
f4 and f5. In a step f6, a sensor starting timer is reset which defines a
time to start the linear A/F sensor S. Thereafter, the sensor starting
timer is started in a step f7.
A next step f8 determines whether the count of the sensor starting timer
exceeds a preset value .theta. which has been set to an interval of time
long enough for the air-fuel ratio sensor to be activated while the engine
is being warmed up. If the count of the sensor starting timer does not
exceed the preset value .theta., then control goes to a step f17 in which
an air-fuel ratio feedback control coefficient KFB is set to 1. Then, the
air-fuel ratio feedback control process is inhibited in a step f18.
Subsequently control proceeds to a step f19 in which a fuel injection rate
Fuel is calculated. Specifically, a rate of fuel to be injected is
determined from a map depending on the engine rotational speed N and the
engine load A/N, and the determined fuel injection rate Fuel is stored in
a predetermined memory area. Stated otherwise, the open-loop process for
controlling the rate of fuel to be injected is carried out in the step
f19. Thereafter, control returns from the step f19 to the main routine. In
a fuel injection routine (not shown) prior to the above process, the rate
of fuel to be injected is determined in response to an interrupt at a
certain crankshaft angle, and fuel is injected at the determined rate to
achieve a target air-fuel ratio determined by the air-fuel ratio open-loop
control process.
Thereafter, since the starter flag is 0 in the step f2, control goes from
the step f2 directly to the step f8. If the count of the sensor starting
timer exceeds the preset value .theta. in the step f8, then control
proceeds to a step f9. In the step f9, if the sensor starting timer is
still in operation, the counting operation thereof is stopped while
retaining the count achieved so far. Then, control goes from the step f9
to a step f10.
The step f10 determines whether the pump cell operation flag is 1 or not.
If the pump cell operation flag is not 1, then control proceeds to a step
f11 in which the pump cell 21 is operated. Then, the pump cell operation
flag is set to 1 in a step f12, which is followed by a step f13 in which a
pump cell operation timer is started. A step f14 determines whether the
count of the pump cell operation timer exceeds a preset value .epsilon.
which has been set to an interval of time long enough for the output
signal of the air-fuel ratio sensor to be stabilized. If the count of the
pump cell operation timer does not exceed the preset value .epsilon., then
control goes to the step f17 for continuing the open-loop control process.
If the count of the pump cell operation timer exceeds the preset value
.epsilon., i.e., if the sensor output becomes stable and the pump current
Ip becomes reliable, then control goes from the step f14 to a step f15. In
the step f15, if the pump cell operation timer is still in operation, the
counting operation thereof is stopped while retaining the count achieved
so far. Then, control goes from the step f15 to a step a20.
The step f20 determines whether the linear A/F sensor S has failed or not.
The step f20 is shown as a subroutine #1 in FIG. 16. The subroutine #1
determines whether the linear A/F sensor S has failed or not based on the
air-fuel ratio signal Vout. If the fuel determining flag F1 is not 1 in a
step g1 and the air-fuel ratio feedback control process is effected in a
step g2, then control goes to a step g3. If the fuel determining flag F1
is 1 in the step g1 and the air-fuel ratio feedback control process is not
effected in the step g2, then control returns to the main routine shown in
FIGS. 15(a) and 15(b).
The step g3 reads a target air-fuel ratio that has already been determined
in the main routine depending on operating conditions of the motor vehicle
which incorporates the air-fuel ratio control system according to the
present embodiment. Then, the air-fuel ratio signal Vout from the linear
A/F sensor S is read in a step g4. A step g5 thereafter converts the
air-fuel ratio signal Vout into an actual air-fuel ratio according to a
predetermined map (not shown) of air-fuel ratios vs. air-fuel ratio
signals.
A step g6 calculates a deviation or error .DELTA.A/F between the target
air-fuel ratio and the detected air-fuel ratio from the air-fuel ratio
sensor. A step g7 then determines whether the error .DELTA.A/F exceeds the
threshold value .alpha. for determining a sensor failure or not. If the
error .DELTA.A/F does not exceed the threshold value .alpha., then control
goes to a step g9. If the error .DELTA.A/F exceeds the threshold value
.alpha., then the fuel determining flag F1 is set to 1 in a step g8.
Thereafter, control returns to the main routine. The step g9 determines
whether the error .DELTA.A/F exceeds the threshold value .pi. or not. If
the error .DELTA.A/F does not exceed the threshold value .pi., control
returns to the main routine. If the error .DELTA.A/F exceeds the threshold
value .pi., then control goes to a step g10 in which the malfunction F2 is
set. Thereafter, control returns to the main routine.
Back in the main routine, control goes to a step f21. The step f21
determines whether the fuel determining flag F1 is 0 or not. If the fuel
determining flag F1 is not 0, it is determined that the error .DELTA.A/F
is in the A/F feedback control interrupt zone or the system shutdown zone.
Control goes to a step f16 in which a pump cell operation stop signal is
applied through the driver 122 to the pump current cutting circuit 14 to
cut off the pump current Ip. Then, control proceeds from the step f16 to
the step f17 for the air-fuel ratio open-loop control process.
If the fuel determining flag F1 is 0, then control proceeds from the step
f21 to a step f22. The step f22 determines whether the present operating
conditions of the motor vehicle fall, within an air-fuel ratio feedback
control range or not. If the present operating conditions are not in the
air-fuel ratio feedback control range, then control goes to the step f17
for the air-fuel ratio open-loop control process.
If the present operating conditions of the motor vehicle are in the
air-fuel ratio feedback control range in the step f22, then control goes
to a step f23. The step f23 determines whether the malfunction flag F2 is
1 or not. If the failure flag F2 is 1, then it is determined that the
error .DELTA.A/F is in the A/F feedback control interrupt zone, and
control goes to the step f17 for the air-fuel ratio open-loop control
process. If the malfunction flag F2 is zero in the step f23, then it is
determined that the error .DELTA.A/F is in the A/F feedback control active
zone, and control goes to a step f24.
The step f24 calculates an actual air-fuel ratio (A/F).sub.2 based on the
air-fuel ratio signal Vout according to the equation: (A/F).sub.2
=f(Vout). Then, the target air-fuel ratio A/F that has already been
determined in the main routine depending on operating conditions of the
motor vehicle is read, and an error or difference .epsilon. between the
read target air-fuel ratio A/F and the actual air-fuel ratio (A/F).sub.2
is calculated, and so is a difference .DELTA..epsilon. between the
presently calculated error .epsilon. and the previously calculated error.
Finally in the step f24, a corrective coefficient K.sub.FB is calculated
for the control of a fuel injection rate based on the air-fuel ratio.
The corrective coefficient K.sub.FB is calculated as the sum of, or
difference between, a proportional term K.sub.A(.epsilon.) of a gain
depending on the level of the error .epsilon., an offset K.sub.p for the
prevention of a response delay owing to the three-way catalytic converter,
a differential term K.sub.D(.DELTA..epsilon.) depending on the difference
.DELTA..crclbar., an integral term .SIGMA.K.sub.I(.epsilon.,tFB), and 1.
Thereafter, control goes to the step f19 in which a proper rate of fuel to
be supplied at the time is calculated from the corrective coefficients
K.sub.FB, K, and the basic fuel injection rate F(A/N,N). Control then
returns to the step fl in the main routine.
The rate of fuel to be supplied which is thus determined in the routine
shown in FIGS. 15(a) and 15(b) is called in the fuel injection routine
that is executed at the time of an interrupt effected in response to a
crankshaft angle signal produced in the main routine. The fuel injection
nozzle N is then actuated by the driver 121 for an interval of time
corresponding to the determined rate of fuel to be supplied, thereby
injecting fuel at the rate which achieves the desired air-fuel ratio.
Although certain preferred embodiments of the present invention have been
shown and described in detail, it should be understood that various
changes and modifications may be made therein without departing from the
scope of the appended claims.
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